EP0637570A1

EP0637570A1 - Treatment of solid and liquid effluents from tan-yards for removing and recovering the chromium contained in the effluents

Info

Publication number: EP0637570A1
Application number: EP94906933A
Authority: EP
Inventors: Sebastián AMER AMEZAGA; Guillermo Amutio Polo; Jaime Centro Investigacion Y Desarrollo Cot Cosp; Antonio Cent. Nac. Invt. De La Cuadra Herrera; José Luis Centro Nacional Investigaci LIMPO GIL; Angel Centro Nac. Invt. Metulorgicas Luis Martin; Alberto Centro Invest. Y Desarrollo Manich Bou
Original assignee: Consejo Superior de Investigaciones Cientificas CSIC
Current assignee: Consejo Superior de Investigaciones Cientificas CSIC
Priority date: 1993-02-11
Filing date: 1994-02-11
Publication date: 1995-02-08
Anticipated expiration: 2014-02-11
Also published as: DE69412728T2; ATE170163T1; DE69412728D1; EP0637570B1; WO1994018133A1

Abstract

The present invention relates to the integral treatment of chromium containing wastes from tan-yards according to the diagram of figure (1). There is provided one or two solid (1) and/or liquid (2) effluents which contain chromium. The solid effluent is subjected to a treatment (4) with perborate (3) producing a chromium-free residue (5) which is treated by conventional methods (for example hydrolysis in order to produce gums or chelagen) and waters which contain the chromium present as Cr(VI). Said Cr(VI) is reduced to Cr(III) by redution in acid medium at (6). In a reactor (7) the solution containing Cr(III) is treated continuously with an alkaline solution (8), to give waters with less than 1 ppm of Cr(9) and a pulp (10) containing Cr(OH)₃.3H₂O from which is obtained, by acidification with sulphuric acid, a concentrated solution of chromium sulphate reusable in the same tan-yard. The present invention provides for the precipitation of Cr(III) contained in solutions, by using magnesium hydroxide (or mixture with NaOH); the chromium-containing solution is slowly and continuously added to another solution, maintained at a temperature between 60 and 63ºC, to which is also added the neutralizing agent. Thereby, the precipitate obtained has particular characteristics of solubility and settling which cannot be obtained in a precipitate produced with other neutralizers. In the solid wastes, the oxidation is performed by stirring these chopped wastes with an aqueous solution which contains a perborate in the proportion of at least 1.5 mol of BO₃ per at-g of Cr contained in the residues (for example 2.35 g of NaBO₃ per g of Cr), during a period of time longer than 10 minutes, the recommended amount of perborate to be used, if sodic salt is used, being from 3.3 to 3.7 g of NaBO₃ per g of Cr contained in the residue to be treated and the recommeded treatment time being 45 minutes. The perborate may be substituted by a mixture of oxygenated water, H₂O₂, and a borate (for example borax) in a solution whose pH must be comprised between 7.5 and 11.5, preferably between 8.5 and 9.5.

Description

Introduction

The tanning industry is considered as the cause of a considerable amount of pollution due to both its solid and liquid waste; a typical percentage distribution of the different effluents might be the following:

Purifying plant sludges 35.0%

Fats 30.0%

Cutoff chippings or waste 10.0%

Fleshing and gristle 8.1%

Shavings 6.0%

Trash (wood, plastics, etc.) 10.9%
Regarding solid waste, the partial percentages for different by-products varies widely according to the type of manufacturing (ovine, capric, bovine, etc.). As in the above case, and referring to the production of medium-fine skins (lamb), the following percentage distribution may be found:

Fleshed and dry skin (for tanning) 23%

Clean and dry wool (saleable) 20%

Rough skin cuttings 17%

Proteins dispersed in drainage water 15%

Insoluble matter 25%
Cuttings have the following distribution:

Collagen 27%

Wool 32%

Fatty matter 15%

Others 26%
Proteins and insoluble matter are dumped into the sewage water from the tanning industry; insoluble matter (made up of fats, collagenic proteins and suety keratinics) and are in suspension in this water. The global DQO for these drainage waters is 2 500 ppm with a pH of 8.
The amount of water required (and therefore dumped) by the said industry is about 30 m³/day.
The above problems worsen due to the almost generalised presence of chromium in cutoffs and shavings (coming from the tanning).
It is well known that in the last few years the tanning industries have been increasingly required to start with skins in a "wet-blue" state that generates greater quantities of this solid waste, composed 90% of proteins and 5 to 6% of chromium oxide.
With regard to the chromium content in the water, it is known that Cr(VI) is 100 times more toxic for mammals than Cr(III). It has been proved that at this valency, chromium is essential for many of the functions of the organism (it increases the action of insulin and retards arteriosclerosis) and its daily requirement is 62 µg/L, a larger amount than the average of 15 µg/L found during analyses of water. In any event, the content of Cr(III) in drainage water must be less than 2 mg/L.
At this time Cr(VI) which previously was widely used in two bath tanning, has practically disappeared from tan-yards. On the other hand, Cr(III) is currently the most important tanning agent and attempts to find a substitute for it have been without success. This means that chromium with this valency appears in important concentrations in tanning bath waste. Furthermore, the washing of wet-blues gives rise to the appearance of some 30 to 50 mg/L of chromium in drainage water, a figure that may fall to 6-10 mg/L in some cases but which in any event requires appropriate treatment to reduce these contents to preferably less than 1 mg/L.
The two most important operations carried out with the skins are: washing and tanning. When the skin is ready the tanning process begins where the Collagen structure is stabilized using natural or synthetic tanning products. The most widely used process is chromium tanning, with a process that may last between 4 and 24 hours.
The tanning product used is a complex hydrous sulphate of chromium III (with 22-25% Cr₂O₃) in an 8-12% preparation with 1 % bicarbonate of soda as the base agent. Vegetable tanning is practically nonexistent and is only used in very special cases such as the preparation of skins for harness-making.
Products obtained during this process have a chromium content (expressed as Cr₂O₃) of about 3%. During the manufacture of products using tanned materials, an appreciable amount of waste skin is produced which because of its high chromium content should be sent to controlled dumps, with the ecological problems that this causes.
Although non-chromed by-products are easily disposed of for the preparation of different types of industrial glues and/or gelatins, chromed by-products are not acceptable for these, mainly because:

1) The presence of Cr(III), the Collagen tanning agent, acts to stabilize the structure of these by-products, making them more resistant to conditions conventionally used for the preparation of these glues and/or gelatins.
2) An attempt has been made to overcome the above problem by strengthening treatment conditions, for example by an increase in temperature and the modification of other variables, so that the protein becomes soluble; however, it continues to hold a sufficient amount of chromium to make it unusable in the foodstuffs field and, in general, of little use in the field of industrial glues since the residual chromium modifies their properties.

State of the Art

The tanning industry carries out research to eliminate to the maximum the chromium present in solid by-products. Processes have been suggested that use specific types of enzymes that act in the presence of Cr(III). A method has been patented in the United States that is based on the use of a 10% solution of sodium glycolate which acts at 40 °C at a pH of between 8 and 9 in two consecutive operations, following which the washed residue is treated with a 10% solution of NaOH with 20% Na₂SO₄; the washed residue is then treated with a solution containing 5% of Na₂SO₄ and 5% of H₂O₂; the pH is adjusted to 8 - 8,5 and the gelatine extracted with a 1 % solution of magnesium oxide.
The greater part of the work carried out over the last few years, on the processing of solid wastes generated by the tanning industry, has a common factor, i.e. they apply a similar methodology with different details. For example, when attempting to process chromed by-products they are first dechromed, generally in the following manners: sequestering action of sodium glycolate (mentioned earlier), alkaline hydrolysis (with sodium hydroxide, calcic, magnesium oxide) or direct attack by enzymes; later, using a series of stages such as filtering, neutralizations and purifications, a protein hydrolysate is obtained; finally gelatine, animal glue, fertilizers, synthetic agglomerates, etc. are obtained. The treatment and revaluation of fleshing products by hydrolytic means is described in the following works:

R. Lynton et al., JALCA, 22, 301 (1982)
Iv. Goschev, Iv. Botev, P. Nedkov, Das Leder, 19, 56 (1985)
P. Bataille and F. Gagnon, JALCA, 78, 328 (1983)
G. Guardini, JALCA, 81, 78 (1986)
J.Zuchowski, JSLTC, 71, 15 (1986)
W. Pauckner, Tech. Report, AIICA, n° 3, 8-16 (1987)
A. Simoncini, G. de Simone, M. Tomaselli and G. Ummarino, Tech. Report, AIICA, n° 3, 17-27 (1987).

During the XXI Congress of the International Union of Leather Technologists and Chemists Societies (IULTCS) held in Barcelona on 25-29 September 1991, the following papers were read, related to these subjects:

Ch. Poschenrieder, B. Gunse and J. Barcelo "Chromium, a major Problem Limiting the Agricultural Use of Tannery Sludges".
G. Manzo and B. Naviglio "Semiindustrial Plant for Lipids and Proteins Meals Production from Chromed Residues or its Copolymers with MMA and AN".
J. Mata-Alvares and F. Cecchi, "A Biotechnological Approach to Treat Vegetal Tannery Effluents".
J. Salmeron, "Practical Example of Recuperation of non Tanned Residues".
M. Taylor, E. Difendorf and W. Marmer, "Efficiency of the Enzymatic Solubilization of Chrome Shavings as influenced by Choice of Alkalinity-inducing Agents".

Spanish patent ES 538759 carries out the operation to eliminate chromium without the need to use sodium glycolate in shorter times than the above, introducing a grinding operation that facilitates the attack by hydrogen peroxide in an alkaline medium (6-12% Na₂CO₃) recovering the chromium in the solution, passing this through an ionic exchange column, following the technique described in Spanish patent ES 506254. Spanish patent ES P9200499, titled "Procedure for the integral treatment of tanning by-products through controlled attack with hydrogen peroxide" describes the elimination of chromium using treatment with oxygenated water in a salified alkaline medium with sodium sulphate. Both procedures, which have an undoubted technical value, suffer from two fundamental problems: the need to use much higher H₂O₂ contents than the stoichiometric and high carbonate and sulphates contents. The latter considerably increases the treatment cost since, although the ionic exchange could recover the Cr(VI) and recycle the solution, in practice in an alkaline medium the load capacity of Cr(VI) in an ionic exchange column is very small.
In any event, the elimination of chromium from solid wastes ends when this element passes into the solution. Furthermore, chroming operations which, as mentioned above, give rise to an important flow of water with high chromium contents, means that the elimination of this element from the water, either from tan-yards or from factories that treat the chromed products, is an inescapable problem for which various solutions have been proposed.
In view of the difficulty arising, for kinetic reasons, when using ionic exchange techniques, in order to be able to use these the oxidation of Cr(III) to Cr(VI) (Spanish patent ES 506.254) has been proposed, which is very easily fixed using ion exchanger resins which are eluted with a reducing solution (hydrogen peroxide in an acid medium) that changes the chromium back to Cr(III). McClellan et al. [B.E. McClellan, M.K. Meredith, R. Parmelle and J.P. Beck, Ana. Chem., 46(2), 306(1972)] describe the extraction of Cr(III) with amine trioctyl, using thiocyanate as the sequester of the chromium obtaining a refined product with less than 0.02 ppm of Cr but at a cost, as mentioned above, of a residence time of almost three hours.
A conventional method for separating Chromium(III) from its solutions is by precipitating it as hydroxide:

Cr³⁺ + 3OH → Cr(OH)₃(act)

whose solubility product, according to Kovalenko [P.N. Kovalenko, Ukr. Khim. Zh. 22, 801 (1956)] is 6.16·10⁻³¹.
Bjerrum has studied solubility in an acid medium [J Bjerrum, Z. Phys. Chem. 73, 724-59 (1910)] and Fricke and Windhausen in an alkaline medium [R. Fricke and O. Windhausen, Z. anarg. allgem. Chem. 132, 273-88 (1924)] for orthorhombic violet hydroxide, with formula Cr(OH)₃.nH₂O to which they assign a value of 3 for n.
Both in scientific publications and in patents, various reagents are described for use to precipitate the hydroxide, among them sodium hydroxide, ammonium and oxides (or hydroxides) of calcium and magnesium. The use of the latter compound, described in 1982 [K. Seubert and A. Schmidt, Lieb. Ann., 267, 218/48, 239 (1982)] has been proposed as the best by Langerwef and Wijs [J.S.A. Langerwef and J.C. de Wijs, Das Leder, 28(1), 1 (1977)] in a bibliographic review of different ways of eliminating chromium from drainage waters from tan-yards.
The main problem for precipitating chromium in these drainage waters is the presence in them of sequester anions of Cr(III), principally formiates and glutamates. These ligands (represented here as L), when strongly joined with Cr³⁺ according to the reaction

^o Cr³⁺ + nL⁻ ¬ CrL_n ^3-n,

noticeably affect the solubility of the hydroxide.
This sequestering effect is used in US patent 4.560.546 (1983) which adds acetic acid or acetates to waste water before precipitation, by neutralization with sodium hydroxide at a pH between 7 and 10, obtaining better results with regard to decantability than when precipitation is carried out by only adding sodium hydroxide, ammonium or ammoniac carbonate as described in a prior patent [US 3.950.131 of 1973].
European patent EP 0003.862 [priority NL 7.802.123 of 25/02/78] also describes the precipitation of heavy metals, particularly chromium, alkalifying the solution up to a pH of between 5 and 10 and a at temperature between 60 and 100 °C.
Costas [D.I. Costas, Ind. Usoara, 33(10), 446 (1986), cited in Chemical Abstract 107, 113, 136253v (1987)] also separates the chromium from the tan-yard effluent using hot neutralization of the solution.
Special attention is drawn to European patent EP 0.341.490 [priority DE 3.815.948 of 10/05/88] which describes the precipitation of chromium in tan-yard drainage water, by the addition of magnesium oxide or hydroxide at a temperature above 50 °C and a pH between 8.2 and 9 (preferably 8.3 and 8.7) which in spite of the presence of carboxylic sequester acids of Cr(III), gives a precipitate rich in chromium, reusable, and a solution with less than 1 mg/L of this element, although to aid in decantation flocculants must be added.

Brief description of the invention

The present invention covers the integral treatment of tan-yard waste containing chromium, according to a diagram described in figure 1.
One or two effluents are taken, solid [1] and/or liquid [2] containing chromium. The solid effluent undergoes treatment [4] with perborate [3], described later, giving rise to a chromium free residue [5] that is treated by conventional methods (for example, hydrolysis to produce gums or chelagen) and waters which contain the chromium present as Cr(VI). This Cr(VI) is reduced to Cr(III) by reduction in an acid medium at [6]. In a reactor [7], also described later, the solution or any other containing Cr(III) is treated continuously with an alkaline solution [8], to give waters with less than 1 ppm of Cr [9] and a pulp [10] containing Cr(OH)₃.3H₂O from which is obtained, by acidification with sulphuric acid, a concentrated solution of chromium sulphate reusable in the same tan-yard.
The present invention provides for the precipitation of the Cr(III) contained in solutions, using magnesium hydroxide but, different from that indicated in the state of the art, the magnesium hydroxide does not necessarily have to be the precipitating agent. The solution containing the chromium is added slowly and continuously to another solution maintained at a temperature above 55°C (preferably between 60 and 63°C) to which the magnesium hydroxide (or better still oxide) is also added continuously, preferably accompanied by a soluble alkali, for example a solution of NaOH. In this way, when precipitating the chromium (III) on the magnesium hydroxide, the precipitate formed has particular solubility and decantability characteristics not present when it is obtained with other neutralizers.
The new aspect of the present invention lies in the fact that the principal mission of the magnesium hydroxide is to act on the characteristics of the precipitate, while its action as a neutralizer is secondary in view of the fact that this second mission can be accomplished, as stated earlier, by the use of a reagent such as sodium hydroxide together with magnesium oxide (or hydroxide).
To carry out this operation, a compartmented reactor is required [6] as shown in figure 2, in which although three compartments have been drawn [6], [7] and [8], only two are required; although in the drawing the system whereby the reagents are mixed, and how the pulp passes from one reactor to another, has been defined, in practice it can be done in other ways, for example adding the neutralizer and the solution to be treated onto the surface of the first reactor and passing the pulp from the first to the second through the inside of the tank, or causing the reagents to reach below the level of the agitator, taking the overflow through a pipe from one reactor to the lower part of the next one, as shown in the drawing.
In this type of reactor, the solution to be treated [1] is introduced by a pump [2] into the first compartment [6] (initially holding only water). Using an appropriate dosage system (for example a hopper dosifier [3] and conveyor belt [4] as shown in the figure), the magnesium oxide (or hydroxide) is introduced and is drawn by a small flow of liquid (which can be a fraction of the treated solution [15]) and is mixed at a point opposite the feed intake with the solution in this compartment.
The magnesium can be dosed according to the Cr content in the feed solution. However, as this content may vary over the time that the operation lasts, it is best to regulate the addition of magnesium [MgO or Mg(OH)₂] by placing in this compartment a pH gauge [9] on which the set point may be between 7 and 7.5.
A heating system is included to maintain the temperature, measured by a thermometric sensor [10], above 55°C.
Finally, in order to obtain a concentration of Cr lower than 1 mg/L, the volume of each compartment must be such that the residence time of the solution (defined as the ratio between the reactor volume and the flow of solution passing through it) in each one of them is above a specific figure which is between 2 and 4 hours.
The following operations to be carried out are totally conventional: the pulp leaving the reactor goes to a decanter [11] from which the overflow is the purified solution [15], which may be recycled in part to aid in introducing the neutralizer through [5]. The thickened product can be taken to a reactor [12] where, it is re-dissolved using sulphuric acid [13] to obtain an acid solution [14] which, appropriately neutralized, can be used again in the tan-yard.
Having solved the elimination of Cr(III) from tan-yard waters up to contents of less than 1 mg/L, it is more appropriate for the solution containing the Cr(VI) obtained from the oxidation of the chromium contained in the skins, to have the least possible salts content so that the reduction of Cr(VI) to Cr(III), which must be carried out in an acid medium, requires the lowest possible consumption of reagents.
The use of hydrogen peroxide to oxidize and solubilize the chromium set in the skin, has been described in the state of the art, but offers the problem of its high reactivity which demands the use of an important excess over the stoichiometry and the addition of a high content of alkaline salts (sodium carbonate) to the attack solution. The present invention proposes the substitution of oxygenated water by a soluble perborate (sodium) which, because of the equation

BO₃⁻ + H₂O ¬ BO₂⁻ + H₂O₂

is able to contribute oxygenated water in sufficient concentration to oxidize the chromium without noticeably affecting the organic material, so that practically stoichiometric oxidant consumptions can be achieved with respect to the chromium, and the high contents of alkalis eliminated that make difficult the later precipitation of the chromium in the solution, according to the present invention.
Furthermore, the freed borate ion, according to the equation

BO₂⁻ + H⁺ ¬ HBO₂

blocked the solution in an appropriate manner so that the acid generated by the reaction

3BO₃⁻ + Cr₂O₃ + 2H₂O → 2CrO₄²⁻ + BO₂ + 4H⁺

makes it unnecessary to use any alkalinizer.
The greater cost of the perborate over oxygenated water is more than compensated for by the greater output by the reagent and the saving in alkalinizer.
Taking into account that the paste is soaked with the chromate solution, the amount of chromium retained can be decreased by an attack in counter-current, as shown in the diagram in figure 3, where [1] represents the reactor, [2] the solid/liquid separation system, [3] the lung tank and [4] the conditioning tank.
The attack on the product to be freed of chromium, is carried out in n stages in counter-current with recycling of the solution during each stage; during each of them a fraction of the solution is removed and sent to the mixer [4] where the perborate (or equivalent reagents) is added and the pH adjusted, returning the same volume of enriched solution. During the first stage the material, P, is received, optionally treated with formol, and the solution contains only the minimum perborate corresponding to the stoichiometry of the reaction; in this case, a volume equivalent to the enriched solution received is eliminated from the process as effluent, E, containing the chromium extracted from the product. During the last stage, n, the recycled solution is increased with washing water.
In the mixer [4] are brought together the volume from the n stages with the necessary reagents, R, to keep the solution at a pH of between 8.5 and 10, and with a perborate (or better, borax and oxygenated water) content of at least twice the minimum corresponding to the stoichiometry of the reaction. The ratio of solution to solid in the reactor [1] must be more than 2, preferably between 14 and 16 times the weight of dry solid.
Figure 4 shows in greater detail the recommended solution for which only two stages are used: the material from which the chromium is to be removed [10], chopped, is taken
Figure 4 shows in greater detail the recommended solution for which only two stages are used: the material from which the chromium is to be removed [10], chopped, is taken to a tank [1] where it is treated with a solution [13] of formol for preparation. The greater part of this solution is used after verifying in [2] the solid/liquid separation, and it is only necessary to add the water [11] and formol [12] that draws the paste [14].
The treatment with perborate is carried out in two stages in order to achieve greater efficiency in the chromium solubilization and, over all, so that the amount of Cr(VI) that is soaked into the paste [24] is reduced to the minimum, while the concentration of Cr(VI) in the drainage [17] is the maximum.
The stabilized product goes to a reactor [3] where it is treated with a recycled solution [16] and another [15] enriched with perborate; the pulp separates from the solution in [4] of which a fraction is the process effluent from which the chromium is recovered, while the other is [16] which is recycled. The paste goes to a second reactor [6] where it is treated with a part of the recycled solution [21] and the remainder [15] of the enriched solution; the pulp is separated from the solution in [7] and washed with water [24], leaving a chrome-free paste [25] and a solution which is a mixture of the filtrate and the washing water which is recycled. The remainder of the recycled solution [22] is received in a mixer [5]; in the solution the perborate [19], or the reagents to obtain it, is dissolved and the pH of the solution adjusted to 10 with soda or another alkali [20].
Note: We give the name pulp to a fluid mixture of solids and liquids (normally 5 to 10% solids) and paste to the wet solid (generally with 40% water) obtained following centrifugation.

Detailed description of the invention

Below we shall describe separately the present invention, commencing with the treatment of liquid effluents (that in any case is essential) and afterwards that for solid effluents.

Treatment of liquid effluents

With reference to the treatment of liquid effluents, the present invention is based on the solution from which the chromium must be removed, continuously reaching a pulp. In this pulp, the solid phase is formed essentially by the precipitated chromium hydroxide, with which a certain amount of magnesium hydroxide is maintained, either by adding this latter product or by its formation in situ from magnesium oxide.
The pH of the pulp may be set at any figure above 7, and the upper limit is not a constraint; the higher it is, the greater the excess magnesium oxide (or hydroxide) to be used. It has been found that a figure of 7.5 as the upper limit is sufficient.
To obtain a residual solution with a Cr(III) content less than 1 mg/L and a morphology that allows good decantation of the precipitate, the following operational conditions are required:

1 - The reactor in which the precipitation takes place must, as the reagent begin to reach it, be filled with an aqueous solution at more than 55°C (it has been found sufficient to hold it at 60°C ± 3°C). Although the precipitation reaction is exothermal [ΔH = -161.2 kJ/mol], a heating system must be available to maintain this temperature in the first reactor. In the second (and the third if used) reactor, heat insulation is sufficient to compensate for losses with the exterior.
2 - That when the reaction begins, the reagents (solution to be treated and precipitate) run continuously onto the water or previously treated solution.
3 - That there is Mg(OH)₂ in suspension at all times, which leads to two possible operating conditions:
- a) The use of excess reagent above that corresponding to the reaction stoichiometry (condition that pertains if the magnesium is added as oxide and the pH is held above 7, and allowing a residence time in each reactor of 2 to 4 hours), or
- b) Add a fixed amount of magnesium oxide (0.3 to 0.7 g per L of solution to be treated) while recycling part of the precipitate obtained (preferably more than 50%), and holding the pH in the first reactor between 7 and 7.5 by the continuous addition of a solution of soluble alkali (for example sodium hydroxide).

The latter alternative allows both a substantial decrease in the amount of magnesium oxide (between 5 and 10 times less, for a solution to be treated having some 5 to 7 g/L of Cr), and also a decrease in the required purity of the oxide. Figure 2 shows the introduction of the soluble alkali [16] using the dosing pump [17], while at the same time part of the chromium hydroxide precipitate obtained is recycled, pumped by [18].

Treatment of solid effluents

Oxidation with sodium perborate has shown itself to be more effective than oxygenated water, and no alkalinizer need be used since the borate formed during the reaction gives sufficient alkalinity. It has been found that, without the need to heat the solution and using excess perborate of less than 50% of the stoichiometric according to the reaction [1], solubilization of more than 95% of the chromium is achieved. This reaction takes place fast so that in 10 minutes appreciable chromium solubilization has taken place, although the recommended time to reach solubilization of 95% of the chromium is 45 minutes.
The reaction that determines the reaction stoichiometry, presuming that the chromium is present in the skins as Cr₂O₃, is:

3BO₃⁻ + BO₂ + Cr₂O₃ + 2H₂O→2CrO₄²⁻ + 4HBO₂ [1]

in which, as can be seen, the presence of 1/3 borate is required which in turn blocks the solution according to the equation:

which borate is formed in the neutralization of recycled boric acid, format [1]
Although in the present invention the recycling of the solution is not essential, this operation is highly recommended for the reasons given below. If this recycling is not carried out, it has been found that borate is always present without the need to add it, either from the decomposition of the excess perborate itself:

BO₃⁻ → ½O₂ + BO₂ [3]

or from the reaction of the perborate with the formol added to stabilize the organic material:

2BO₃⁻ + HCHO→CO₂ + 2BO₂⁻ + H₂O [4]

Recycling of the solution permits both the obtaining of chromium in the form of a more concentrated solution and a decrease in the volume of effluents from the plant, and as a consequence a reduction in the equipment required to recover chromium from the effluent and comply with the corresponding environmental specifications.
As indicated when describing the block diagram in figure 4, when recycling the solution a preparation tank [5] must be available in which to replace the perborate consumed during the reaction or, as stated, it must be produced "in situ" by conventional processes.
It is well known [see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley, New York 1978-1982. Vol. 17, page 7] that perborate is obtained from borax by its reaction with oxygenated water:

Na₂B₄O₇·10H₂O + 2NaOH + 4H₂O₂ + H₂O → 4NaBO₃·4H₂O [5]

As can be seen from equation [5], soda must be added in order to form perborate. Within the process subject of the present invention, this addition leads to the danger that due to an excess of alkalinity the perborate may decompose since, according to Smith and Martel [R.M. Smith and A.E. Martell, Critical Stability Constants, vol. 4, Penum Press, New York 1976] it has the following equations

BO₂⁻ + H₂O₂ ¬ BO₃⁻ + H₂O [6]

whose constant at 25°C is 20.9 and

H₂O₂ ¬ HO₂⁻ + H⁺ [7]

left, and the effect of the perborate is lost.
To avoid this it is necessary to have boric acid present in the solution at all times, in the form of the equation

HBO₂ ¬ BO₂⁻ + H⁺ [8]

whose constant 5.8·10⁻¹⁰ means that the solution is blocked at a pH of 9.2, lower than 11.7 which is the admissible limit according to [7].
[Note: At an ionic strength of 1 the value of the constant for equation [8] decreases to 1.41·10⁻⁹ according to which the pH at which the solution is blocked decreases to approximately 8.9].
Since the attack must be carried out in an alkaline medium and the dissociation of the oxygenated water avoided, when starting with mixtures of oxygenated water and borax the pH must be maintained between 8.5 and 10, by the addition of the necessary amounts of sodium hydroxide or other soluble alkali. Preferably between 8.5 and 9.5 and never more than 11.5 or less than 7.5.
In this in situ generation of perborate, the recycling of the solution means an appreciable saving of borax (or borate in general), because the solution already holds an appreciable amount of this salt.
Therefore in the preparation tank [5] perborate can be added or the equivalent amount of oxygenated water, an amount of borate (in any of its commercial forms) less than the stoichiometry and, in this case, sodium hydroxide in sufficient quantity to hold the pH of the solution at between 8.5 and 9.5.
The process for the treatment of solids consists basically of mixing the chromed material (preferably stabilized with formol) with a perborate solution, continuously agitating the pulp, without the need for heating, for a period between 10 minutes and 1 hour (preferably 45 minutes), and thereafter the solution is separated, preferably by centrifuging, the pulp, without the need for heating, for a period between 10 minutes and 1 hour (preferably 45 minutes), and thereafter the solution is separated, preferably by centrifuging, and lastly the chrome-free material is washed with water. In practice, if this simple method is used practical problems may arise, especially with regard to effluents and process costs, that may impede its correct industrial application; for this reason recycling and optimization processes must be used that, while not affecting the essentials of the process, make it more viable. Among the possible different solutions, figure 5 gives a recommended process.
The basic equipment is made up of:

Preparation block:

A. Reception hopper
B. Chopping mill
C. Stabilization tank
D. Basket centrifuge

First attack block:

E. First attack tank
F. First attack lung tank
G. Basket centrifuge

Second attack block:

H. Final attack lung tank
I. Final attack tank
J. Basket centrifuge

Reagent adjustment block:

K. Reagent adjustment tank

Cr recovery block:

L. Acidification and reduction tank
M. Lung tank for Cr(III) solution
N. Neutralization tank
O. Refining tank
P. Decanter

TABLE I

#	Name	#	Name
11	Chromed material	26	Chrome-free paste
12	Formol 40%	27	Solution from 2nd centrifuging
13	Recycled solution	27	Solution from 2nd centrifuging
14	Pulp	28	Borax
15	Washing water	29	Soda 50%
16	Wet paste	30	Oxygenated water 50%
17	Recycled solution for first attack	31	Recharged solution for 2nd attack
18	Overflow solution	31	Recharged solution for 2nd attack
19	Pulp from first attack	32	Sodium bisulphide 30%
20	Recharged solution for first attack	33	Sulphuric acid 98%
20	Recharged solution for first attack	34	Cr(III) solution
21	Paste from first attack	35	Soda 50%
22	Solution from first centrifuge	36	Suspension of Cr(OH)₃
23	Solution for second attack	37	Residual water
24	Pulp from second attack	38	Thick pulp of Cr(OH)₃
25	Washing water	39	Load solution
25	Washing water	40	Recharged solution

TABLE II

#	Name	Kg
12	Formol 40%	9-10
15	Washing water	800 - 1,200
25	Washing water	2.000 - 4.000
28	Borax (Na₂B₄O₇·10H₂O)	2.5 - 4.4
29	Soda (NaOH) 50%	1.5 - 3.0
30	Oxygenated water 50%	3.0 - 5.3
32	30% solution of NaHSO₃	9 - 11

Description of the figures

Figure 1 Block diagram of the invention

1 Solid effluent to be treated
2 Liquid effluent to be treated
3 Perborate solution
4 Dissolving of chromium
5 Chrome-free solid
6 Reduction in acid medium of Cr(VI) to Cr(III)
7 Chromium precipitation
8 Alkalinization
9 Chrome-free water
10 Recoverable chromium hydroxide

Figure 2

Diagram of the process using NaOH or other soluble neutralizer

[1] Solution to be treated
[2] Constant flow pump
[3] Reagent hopper [MgO or Mg(OH)₂]
[4] Reagent dosifier
[5] Reagent and flow water discharge funnel
[6] First reactor compartment
[7] Second reactor compartment
[8] Third reactor compartment (optional)
[9] pH gauge
[10] Temperature gauge
[11] Thickener
[12] Pulp re-dissolving tank
[13] Concentrated sulphuric acid
[14] Outlet for concentrated solution of chromium (III) sulphate
[15] Purified solution
[16] Solution of NaOH or other neutralizer soluble in water
[17] Dosing pump for neutralizer solution
[18] Pulp recycling pump

Figure 3

General Diagram of solids treatment

I, II, n-1, m Stages
P Product to be treated
E Aqueous effluent
A Washing water
D Chrome-free product
1 Reactor
2 Solid/liquid separation
3 Lung tank
4 Mixing tank

Figure 4

Block diagram of solids treatment process in two stages

Equipment:

1. Material preparation
2. S/L separation
3. First attack with perborate
4. S/L separation
5. Solution regeneration
6. Second attack with perborate
7. S/L separation

Flows

10. Material for chromium removal	18. Paste from 1st treatment
11. Water	19. Perborate
12. Formol	(20. Borate + oxygenated water)
13. Recycled solution	21. Attack solution
14. Stabilized paste	22. Recycled solution
15. Attack solution	23. Washing water
16. Recycled solution
17. Liquid effluent with Cr

Figure 5

Layout of experimental device used in examples 1 to 6

[1] Jacketed 2 L reactor
[2] Jacketed 2 L reactor
[3] Decantation receptacle
[4] 4 mm diam. solution intake pipe
[5] 10 cm diam. magnesium intake nozzle
[6] 4 mm diam. solution overflow pipe
[7] 4 mm diam. solution overflow pipe
[8] Sampling device
[9] 4 mm diam. solution overflow pipe

Figure 6

Decantation curve for chromium (III) hydroxide obtained

Figure 7

Thermogravimetric curve for chromium (III) hydroxide obtained

Figure 8

Layout of preparation (Example 8)

Equipment:

A. Reception hopper	K. Reagent adjustment tank
B. Chopping mill	L. Acidification and reduction tank
C. Stabilization tank	M. Cr(III) solution tank
D. Basket centrifuge	N. Neutralization tank
E. First attack tank	O. Refining tank
F. First attack lung tank	P. Decanter
G. Basket centrifuge	Q. First washing tank
H. Final attack lung tank	R. Centrifuge
I. Final attack tank	S. Second washing tank
J. Basket centrifuge	T. Centrifuge

Flows:

11 Chromed material	26 Chrome-free paste
12 Formol 40%	27 Solution from 2nd centrifuge
13 Recycled solution	28 Borax
14 Pulp	29 Soda 50%
15 Washing water	30 Oxygenated water 50%
16 Wet paste	31 Recharged solution for 2nd attack
17 Recycled solution for 1st attack	32 Sodium bisulphide 30%
18 Overflow solution	33 Sulphuric acid 98%
19 Pulp from first attack	34 Cr(III) solution
20 Recharged sol. for 1st attack	35 Soda 50%
21 Paste from first attack	36 Suspension of Cr(OH)₃
22 Sol. from 1st centrifuge	37 Residual water
23 Solution for 2nd attack	38 Thick Cr(OH)₃ pulp
24 Pulp from 2nd attack	39 Solution for charging
25 Washing water	40 Recharged solution

Examples

For the water treatment a magnesium has been used that corresponds to a product with the quality of reagent for analysis under the Probus® brand, with a minimum MgO content of 97%, and maximum iron 0.01% and heavy metals (expressed in Pb) of 0.003%. Figure 5 shows the device, at laboratory scale, used to carry out the tests described in the following examples. It consists of two glass reactors [1] and [2] with their corresponding heating jackets to thermostatize the solution; the useful volume of each reactor is 2.1 L. Each reactor has a blade agitator 42 mm in diameter that is turned by a motor (not shown in the figure) at 1000 rpm.
The solution to be treated reaches the reactor [1] impelled by a dosing pump and is placed in the bottom of the reactor by a pipe [4]. The magnesium (MgO) enters every 5 minutes in weighed amounts through a nozzle [5] and is dragged from the walls by a distilled water piston in a syringe so that the volume injected following each addition of magnesium, is known. The pulp overflows at [6] and passes to the bottom of the reactor [2] from where it passes through [7] and reaches a 5 L receptacle [3] that acts as a decanter, with the purified solution exiting via the overflow [9]. In [7] there is a lateral pipeline [8] with its end clipped, for removing samples periodically.

Example n° 1. Test with MgO alone

The operating conditions were:
Temperature in reactors: 60 - 62 °C
Solution flow: 1 L/h
Magnesium added: 0.45 g every 5 min
Flow water: 8 mL every 5 min (0.096 L/h)
Residence time in each reactor: 2.08 h
pH in first reactor: 6.7 to 7.0
Result: Concentration of Cr(III) in sampler [8] filtered: 8.3 mg/L

Example n° 2. Test with MgO alone

The following operating conditions were used:
Temperature in reactors: 60 - 62 °C
Solution flow: 1 L/h
Magnesium added: 0.63 g every 5 min
Flow water: 8 mL every 5 min (0.096 L/h)
Residence time in each reactor: 2.08 h
pH in first reactor: 7.1 to 7.3
Result: Concentration of Cr(III) in sampler [8] filtered: 3.9 mg/L

Example n° 3. Test with MgO alone

Operating conditions:
Temperature in reactors: 60 - 62 °C
Solution flow: 0.6 L/h
Magnesium added: 0.45 g every 5 min
Flow water: 8 mL every 5 min (0.096 L/h)
Residence time in each reactor: 3.2 h
pH in first reactor: 7.25 to 7.6
Result: Concentration of Cr(III) in sampler [8] filtered: 0.6 mg/L
Since the test was carried out beginning with the first reactor charged with water and the second reactor empty, it is appropriate to discover by analysis of the magnesium when the stationary state is reached. Table II gives the contents of Cr and Mg in a pulp taken from the outlet from the second reactor after sedimentation over 2 hours and filtering of the floating liquid.
The decantability of the pulp coming from the second reactor was measured and the curve obtained that is shown in figure 6. It should be noted that it is difficult to measure the height at which the separation surface occurs, indicated by a shaded area; in any event the floating liquid appears cloudy.
The speed of sedimentation, 36 mm/min (2160 mm/h) is almost 10 times greater than that indicated in the Wijs revision (op.cit.) of 250 mm/h.
Analyzing the cloudy product, a chromium content of 2 mg/L is found which decreases to 0.6 on filtering the solution. TABLE III

Evolution of the Cr and Mg at the reactor outlet

Time, h Cr, mg/L Mg, g/L

3^. 0.58 1.13

4 0.70 1.28

5 0.70 1.56

6 0.60 1.71

7 0.75 1.93

8 0.60 2.10

10 0.60 2.08

^. Pulp begins to flow from 2nd reactor
Solids content in thick pulp: 34%
The thick pulp, filtered and dried at 105°C, had the following composition:

Cr 24.5%

Mg 18.0%
Figure 7 shows the thermogravimetric curve.
The X-ray diagramme shows no line to allow its crystalline identification.

Example 4. Test with sodium hydroxide and magnesium oxide

Using the same solutions as in the previous examples, the test was repeated under equivalent conditions to those in example 3. When the second reactor began to overflow (3 hours after beginning the test) the chromium content in the filtered solution was 0.2 mg. From that moment the addition of magnesium was decreased from 0.31 g every 5 minutes to 0.10 g every 15 minutes; the pulp flowing from the reactor was collected every 15 minutes in a receptacle where it was left to sediment, returning the solid residue from one of each of the two receptacles (i.e. 50% of the precipitate obtained) to the first reactor. Simultaneously a solution 2N of NaOH began to be pumped to the first reactor, adjusting the flow so that the pH in the first reactor was held at between 7.2 and 7.5. The result obtained is shown in Table IV: TABLE IV

Evolution of the Cr and Mg at the reactor outlet

Time, h Cr, mg/L Mg, g/L

3 0.2

6 0.7 0.80

10 1.4 0.39

12 2.1 0.32

13 2.7 0.29

15 2.5 0.29

18 2.6 0.27
Soda consumption was: 1.08g/min <> 0.06 l/h
Solids content in thickened pulp: 26.7%

Example 5. Influence of the amount of magnesium added

After 18 hours of testing example 4, the addition of magnesium was double, while the rest of the operating conditions remained the same.
Table V gives the figures obtained (times are counted starting with test 4). TABLE V

Evolution of the Cr and Mg at the reactor outlet

Time, h Cr, mg/L Mg, g/L

20 1.8 0.38

22 1.9 0.44

24 1.8 0.46

26 1.9 0.48

28 1.8 0.51

Example 6. Influence of the recycling of hydroxide

After 28 h of testing example 10 the recycling of chromium hydroxide was eliminated, maintaining all other operating conditions.
Table VI shows the figures obtained (times counted from the end of test 5) TABLE VI

Evolution of the Cr and Mg at the reactor outlet

Time, h Cr, mg/L

30 2.8

32 3.25

34 3.15

36 3.2

37 3.1

38 3.1

Example 7. Direct treatment of solid residues

This begins with chopped material containing 3% chromium (expressed as Cr₂O₃). One kilo of this product is taken and repulped in 15 L of water to which are added 50 g of formol and sufficient sodium bicarbonate, NaHCO₃, to obtain a pH of 7. Thereafter 300 g of perborate sodium tetrahydrate Q.P., NaBO₃.4H₂O, are added.
The mixture is agitated for half an hour and the pulp centrifuged, washing the paste with water, without removing it from the centrifuge, until the Cr(VI) cannot be detected from its yellow colour in the washing water.
The product obtained, dried at 105°C, gave a Cr₂O₃ content of less than 0.01%.

Example 8. Treatment with recycled solutions

The same raw material was used as for the previous example. In a receptacle 15 L of a solution were prepared containing 54.5 g of formol (136 g of solution at 40%) and a pH 7.0 achieved by neutralization of the free acid with sodium bicarbonate. This solution is mixed (according to the layout in figure 8) in an agitation reactor C with 1 kg of raw material to be treated [11]. After 15 minutes of agitation, the pulp is discharged into a basket centrifuge D where the retained solid is washed with 900 ml of water [15], collecting 1.9 kg of wet solid [16] and 15 L of solution that is reused to treat a new amount of product, after adding, [12], 11 g of formol (27.5 solution at 40%) to compensate for the formol drawn off by the wet product, and sodium bicarbonate to return to pH to 7.0.
This product is treated as follows: Three agitator reactors are used, E and I with 20 L capacity, and another K of 6 L, and two storage receptacles F and G with 20 L capacity, and two basket centrifuges H and J. For the first operation the tank E is loaded with a solution with the following composition:

NaBO₃ 350 g

NaBO₂ 40 g

HBO₂ 105 g

Na₂CrO₄ 290 g

Water up to 17 L

and in tank I a solution containing:

NaBO₃ 224 g

NaBO₂ 9 g

HBO₂ 30 g

Na₂CrO₄ 100 g

Water up to 14 L
The stabilized paste [16] from the stabilization stage is added to tank E. This is agitated for 45 min and the pulp [19] passed to the basket centrifuge H, sending the solution [22] to tank F and the paste [21] to tank I, where it is agitated for 45 min, and thereafter the pulp [24] is centrifuged by J and the paste washed in 3 L of water [25]. The solution and washing water [27] are sent to tank G.
The following operations are carried out in the following manner:
* From tank G 25% of its volume is taken [39], which is discharged into K and the remaining 75% [23] into I.
* From tank F 14.5 L are taken [17], which are discharged into E and the remainder [18] is eliminated as effluent (for chromium recovery).
* To tank K is added in this order:

[28] Borax (Na₂B₄O₇.10H₂O) 130 g

[30] Oxygenated water (H₂O₂) 110 vol. 265 cc

[29] Soda (NaOH), 50% solution up to pH=9.5

(Note: do not add the oxygenated water until all the borax is dissolved, heating slightly if necessary; once a pH of 9 is reached, add the soda slowly so that the pH does not rise above 10).
* From tank K 25% of its volume is taken [31], which is discharged into tank I and the remaining 75% [20] is discharged into tank E.
* New stabilized paste [16] is loaded into tank E and agitated for 45 min and centrifuged. The solution [22] goes to F and the paste [21] to I where, with the original solution, it is agitated for 45 min and centrifuged and washed with 3L of water [25], sending the solution [27] to G.
The operation is repeated as often as necessary. The product obtained [26] gives the following analysis:

Attack N° Cr in the pulp, % Na₂CrO₄ in effluent [18] g/L

1 washing [26] 3 washes with repulping

1 0.12 0.05 18

2 0.11 0.06 17

3 0.13 0.05 18

Claims

Treatment of solid and liquid effluents from tan-yards for removing and recovering the contained chromium, characterized because the chromium is solubilized, in the case of solid residues by attack with a solution of perborate and, once in solution and reduced to Cr(III), it is precipitated in the form of hydroxide, mixed continuously in an agitator reactor, initially filled with water, which is held at a temperature above 55°C (preferably between 60 and 63°C), to which is also continuously added magnesium hydroxide (or better oxide), sending the pulp obtained to a second reactor, preferably with the same capacity as the first. The average residence time of the solution in each reactor (defined as the ratio between the reactor volume and the volumetric flow) must be between 2 and 4 hours. The addition of the precipitate is regulated by the measurement of the pH in the first reactor; the pH must not be less than 6 nor is it advisable that it be more than 8; preferably 7 to 7.5.
Treatment of solid and liquid effluents from tan-yards for removing and recovering the contained chromium, according to claim 1, characterized because the oxidation is carried out by agitation of the chopped residues with an aqueous solution containing a perborate in a proportion of at least 1.5 mol of BO₃⁻ per at-g of Cr contained in the residues (for example, 2.35 g of NaBO₃ per g of Cr), for a period of more than 10 min, with the recommended amount of perborate used, if sodium salt is used, of 3.3 to 3.7 g of NaBO₃ per g of Cr contained in the residue to be treated, and the recommended attack time is 45 min.
Treatment of solid and liquid effluents from tan-yards for removing and recovering the contained chromium, according to claim 1, characterized because part of the magnesium oxide (or hydroxide) may be substituted by sodium hydroxide or other soluble alkali to hold the pH in the reactor, provided that this substitution is carried out after sufficient precipitate of chromium (III) hydroxide is available to be able to return part of this precipitate to the first reactor, preferably more than 50% of the product formed, and that the amount of magnesium oxide (or hydroxide) added, expressed as pure oxide, is not less than 0.3 g per L of solution to be treated (preferably 0.7 g/L).
Treatment of solid and liquid effluents from tan-yards for removing and recovering the contained chromium, according to claims 1 and 2, characterized because the perborate may be substituted by a mixture of oxygenated water, H₂O₂, and a borate (for example borax) in a solution whose pH must be between 7.5 and 11.5, preferably between 8.5 and 9.5.
Treatment of solid and liquid effluents from tan-yards for removing and recovering the contained chromium, according to claims 1, 2 and 4, characterized because for the attack a proportion of solution to solid residue (dry) is used of more than 2, preferably 15.
Treatment of solid and liquid effluents from tan-yards for removing and recovering the contained chromium, according to claims 1, 2, 4 and 5, characterized because the attack is carried out in two or more stages in which, in each one, the product to be treated is mixed with the alkaline solution of perborate and, when the attack is terminated, the pulp is separated from the solution which passes to the following stage while a part, preferably 75% of its volume, is reused for the attack in this stage and the remainder, except in the first stage (which is constituted by the process effluent from which solubilized chromium is recovered) goes to a preparation tank where the necessary reagents are added to maintain the concentration and pH in the attack tank to which is returned, enriched, the volume removed; the paste, separated from the solution, passes to the next attack stage. During the last stage, following separation of the solid in the solution, the latter is washed with a volume of water equivalent to that eliminated as effluent, and the paste forms the final chrome-free product.
Treatment of solid and liquid effluents from tan-yards for removing and recovering the contained chromium, according to claims 1 to 6, characterized because the chromium contained in the residue to be treated passes to a solution from which the chrome is recovered as Cr(OH)₃.nH₂O, with the effluent remaining with a low saline content.